1. Field of the Invention
This invention relates to an improved process for defluorination of phosphate rock. More specifically, this invention relates to an agent and method for controlling kiln restriction during thermal defluorination of phosphate rock.
2. Description of the Prior Art
The invention is of particular value in producing animal feed ingredients from mineral phosphates which usually occur in the form of apatite or fluorapatite. The more important natural deposits of mineral phosphates in the United States contain the desirable constitutents calcium and phosphorus in the form of apatite or fluorapatite, the formula of which may be written as 3Ca.sub.3 (PO.sub.4).sub.2 --CaF.sub.2 or Ca.sub.10 F.sub.2 (PO.sub.4).sub.6 or 4CaO.P.sub.2 O.sub.5.F.sub.2. According to these formulae the phosphate rock, if composed wholly of fluorapatite, contains about 3.77% fluorine. Concentrated Tennessee brown rock contains between about 3.0 and about 3.5% fluorine. On the other hand, Florida pebble phosphate rock contains about 3.8% fluorine.
The amounts of combined fluorine in these various mineral phosphate rocks are so high that if fed to animals in their original mined form would produce deleterious effects. Since the fluorine has been found to be deleterious particularly to cattle, it is necessary that this fluorine be substantially removed from the phosphate rock prior to its use as an animal feed supplement. It has been found that a phosphorus:fluorine weight ratio of greater than 100 in a phosphate feed supplement containing at least about 30% of P.sub.2 O.sub.5 is considered acceptable at the present time.
In the past, animal feeds have been manufactured by mixing phosphate rock, "wet process" phosphoric acid and soda ash and allowing them to react. Typically, in the manufacture of phosphoric acid by the so-called "wet process", phosphate rock is treated with sulfuric acid. The calcium is precipitated as calcium sulfate and some, but not all, of the fluorine is volatilized as fluosilicic acid. Because of the residual fluorine, which is difficult to separate, the resulting phosphoric acid is a poor starting point for preparing mono- and dicalcium phosphates for use in animal feeds.
One well known practice for preparing mono- and dicalcium phosphate for use in animal feeds is to calcine the phosphate rock prior to reaction with the sulfuric acid. This is carried out by heating the comminuted phosphate rock to a temperature of about 1100.degree.-3000.degree. F. for a period of time sufficient to volatilize the fluorine.
E. H. Wight et al., U.S. Pat. No. 2,234,511 discloses a defluorination process wherein grannular superphosphate is fed into an inclined rotary kiln. Hot gases pass upwardly from a burner positioned at the lower end of the kiln thereby calcining (600.degree.-800.degree. C.) the superphosphate and substantially completely removing the fluorine.
The calcining is conveniently carried out in a rotary kiln. However, the process suffers from the disadvantage of solid phase component formation due to high temperature fusion. These components deposit on the kiln walls in the form of rings which effectively reduce the inner diameter of the kiln. When the rings become sufficiently thick, large balls of fused material may be unable to pass through the kiln. These deposits are known as "ring and ball materials" which not only impede kiln draft but can also drastically alter the thermal profile within the kiln. Eventually the kiln must be shut down, cooled and cleaned mechanically, all of which interferes with efficient production and increases costs.
Accordingly, there is a need for an improved thermal defluorination process whereby the formation and deposit of ring and ball materials are substantially eliminated, or whereby they can be removed from the kiln without interrupting the process.
C. A. Butt in U.S. Pat. No. 2,360,197 discloses a process for defluorinating triple superphosphate (impure monocalcium phosphate) which avoids build-up of ball and ring materials. Butt discloses adding a basic alkaline earth compound such as limestone, dolomite, hydrated lime or calcium oxide before calcining at a temperature of 875.degree.-900.degree. C.
C. A. Butt in U.S. Pat. No. 2,442,969, discloses a process for defluorination of ground natural phosphate rock by calcining at about 1450.degree. C. Butt discloses adding MgO, Na.sub.2 O, and K.sub.2 O to ground natural phosphate rock and calcining at 1100.degree.-1300.degree. C. Butt also discloses adding H.sub.3 PO.sub.4, calcium monophosphate, dicalcium phosphate or other non-basic phosphate compounds.
In U.S. Pat. No. 2,565,351, Butt discloses another process wherein H.sub.3 PO.sub.4 or another phosphate material, and sulfuric acid are added before calcining at 1275.degree.-1450.degree. C.
Wm. B. Williams in U.S. Pat. No. 2,997,367 teaches the addition of oxygen bearing calcium or alkali metal salts such as soda ash, sodium sulfate, sodium nitrate, sodium formate, sodium chloride, potassium carbonate, potassium sulfate, lime, limestone, calcium nitrate, and calcium formate to the phosphate rock prior to calcining in order to reduce fluorine content. Williams further discloses calcining in two stages: at 1800.degree.-2700.degree. F. (982.degree.-1482.degree. C.) then at 2700.degree.-3000.degree. F. (1482.degree.-1649.degree. C.).
D. E. Tynan in U.S. Pat. No. 3,058,804 teaches mixing an acid-reacting sodium phosphate salt with phosphate rock. Alkali metal salts to be added include Na.sub.2 CO.sub.3, Na.sub.2 SO.sub.4, NaNO.sub.3, NaOOC, NaCl, K.sub.2 CO.sub.3, and K.sub.2 SO.sub.4. Tynan calcines at 1250.degree. to 1450.degree. C.
O. W. Allen, in U.S. Pat. No. 3,292,995, reduces balling by adding sufficient CaCO.sub.3 to the calciner feed to neutralize free H.sub.2 SO.sub.4. Allen calcines at 2500.degree.-2800.degree. F. (1371.degree.-1538.degree. C.).
A. N. Baumann, U.S. Pat. No. 3,851,086, adds CaCO.sub.3 or NaOH to the calciner feed based on triple superphosphate. Other suitable additives include Na, K and Li carbonates, sulfates, hydroxides and halides. Calcium compounds include limestone, hydrated lime, quicklime and calcium sulfate.
U.S. Pat. No. 4,239,523 to Spada discloses a process for making fertilizers rich in calcium orthophosphate. Calcium phosphate bearing minerals are reacted with ammonium biphosphate prior to being fed into a rotary furnace, wherein the materials react to form calcium orthophosphate.
U.S. Pat. No. 4,152,398 to Larson discloses the addition of sodium carbonate, sodium phosphate, sodium nitrate, sodium formate, sodium chloride and/or acidic sodium salts of phosphoric acid in order to prevent fusion of the phosphate rock in the defluorination calcination process.
U.S. Pat. Nos. 3,101,999 and 3,102,000 to Malley et al. disclose the addition of clay, talc (Mg.sub.3 Si.sub.4 O.sub.10 (OH).sub.2) or diatomacious earth to prevent agglomeration during defluorination calcining.
U.S. Pat. No. 2,478,200 to Maust et al. discloses the addition of aluminum phosphate as a catalyst promoting defluorination of phosphate rock having a low silica content (less than 4%). The low silica content assures no fusion or agglomeration in the calcining kiln.
U.S. Pat. No. 3,907,538 to Hauschild discloses the addition of magnesium compounds, such as magnesium carbonate, magnesium hydroxide and magnesium silicate, prior to calcination defluorination in order to prevent fusion and agglomeration build up on the calcining kiln walls.
There have been further attempts by those skilled in the art to flush defluorination kiln restrictions with the addition of orthophosphate salts, and more specifically, mono-ammonium orthophosphate salts, to the kiln feed as shown in FIG. 1. The orthophosphate salts provide cationically stabilized sources of phosphorus in high concentrations. The phosphorus enters into reaction with the kiln charge at the same temperature that fusion of solid phase compounds begins to take place. Phosphorus, which is volatilized during calcination of the charge, is replaced by condensed orthophosphates. The addition of the orthophosphate salts directly to the kiln hinders, and in some cases reverses, the fusion reaction, resulting in flushing or "shelling out" of wall build-up and dissolving of balls.